Across continuous melt-phase PET polymerisation lines, Weilian Chemical PTA is pre-dried in fluid-bed or packed-bed dryers at 160–180°C for 4–6 h until residual moisture reaches <50 mg/kg. The dried PTA is mixed with ethylene glycol in a paste preparation tank at a glycol-to-PTA molar ratio of 1.15–1.30. Esterification proceeds in a first-stage reactor at 260–270°C and 1.5–2.5 bar(a); released water and excess glycol are removed through a reflux column with a distillation range that controls diethylene glycol formation in the range 1.0–1.4 wt% in the final amorphous melt. Commercial PTA quality parameters relevant to this step include 4-carboxybenzaldehyde below 25 mg/kg and p-toluic acid below 150 mg/kg; higher levels broaden the molecular weight distribution and increase oligomeric by-products. Antimony trioxide catalyst is metered at 150–250 mg/kg as Sb, with phosphoric acid or phosphate ester added as a stabiliser at 20–80 mg/kg P. The prepolymer is transferred to a finisher operating at 285–290°C under vacuum <1 mbar, where intrinsic viscosity measured by ASTM D4603-18 is raised to 0.62–0.65 dL/g for bottle-grade amorphous granules.

The amorphous pellet is crystallised at 170–180°C in a fluidised-bed pre-crystalliser to prevent sticking, then subjected to solid-state polymerisation at 215–220°C under a counter-current nitrogen stream for 8–12 h. The SSP reaction shifts the molecular weight upward through polycondensation of terminal hydroxyl and carboxyl groups, and simultaneously strips acetaldehyde to below 1 mg/kg when measured by ASTM F2013-10. Bottle preform injection moulding requires pellet IV of 0.82–0.85 dL/g, residual moisture below 50 mg/kg, and cylinder temperatures from 270–295°C in an injection unit with a low-shear screw of L/D 20:1–24:1. Stretch-blow moulding of preforms at reheating temperatures of 100–110°C uses axial draw ratios of 2.8–3.2 and hoop draw ratios of 3.5–4.0; this biaxial orientation increases sidewall tensile strength and reduces creep for carbonated soft drink packaging. Food-contact compliance is assessed under FDA 21 CFR 177.1630 and EU 10/2011, with total migration and acetaldehyde release forming part of the release protocol on production-scale preform lines.

What Limits High-Speed Polyester Staple Spinning with PTA-Based Chips?

The conversion of PTA-based PET chips into staple fibre starts with crystallisation at 160–170°C and drying at 170–180°C for 5–8 h to a final moisture level below 50 mg/kg; water above this threshold hydrolyses the ester bond in the extruder and reduces melt viscosity. Fibre-grade PET typically carries an intrinsic viscosity of 0.63–0.68 dL/g measured by ASTM D4603-18, with diethylene glycol content held at 1.0–1.3 wt% to balance dye uptake and thermal stability. Melt extrusion through a single-screw extruder with L/D 24:1–30:1 and compression ratio of 3:1 is followed by melt filtration through a screen pack of 20–40 μm absolute rating; spin packs contain spinneret holes of 0.20–0.30 mm diameter and operate at take-up speeds of 1500–3500 m/min depending on whether partially oriented yarn or fully drawn yarn is targeted. Delusterant TiO₂ is added as a masterbatch at 0.3–0.5 wt% in semi-dull fibres; agglomeration above this level creates spinning pack pressure rise and filament breaks.

The as-spun tow is drawn in multiple stages. Draw ratios of 3.0:1–4.0:1 at roll temperatures from 70–90°C for first-stage draw and 140–170°C for heat setting reduce elongation at break and increase tenacity; final staple fibre tenacity measured to ISO 5079:2020 typically reaches 5.5–6.5 cN/dtex for high-tenacity grades and 3.0–4.0 cN/dtex for regular grades. Spin finish oil pick-up is controlled at 0.25–0.40 wt% using aqueous emulsions applied after quenching; this controls fibre-to-metal friction on draw rolls and electrostatic charge on cutting lines. Crimping is carried out at 100–120°C with a stuffer-box crimper; the crimp frequency of 10–14 crimps/cm determines carding performance in nonwoven and ring-spun yarn downstream. If granulate is exposed to air at relative humidity above 60% for more than 1 h, re-drying is required before extrusion to avoid measured IV loss greater than 0.02 dL/g.

In biaxially oriented PET film manufacturing, PTA-derived polymer is polymerised to a lower intrinsic viscosity of 0.60–0.64 dL/g by ASTM D4603-18, with low carboxyl end-group content and tight oligomer control. The resin is dried to below 50 mg/kg moisture and extruded through a flat die at 275–285°C onto a chill roll held at 25–35°C, where electrostatic pinning suppresses air entrapment between web and chill roll. Sequential drawing at machine direction draw ratios of 3.2–3.6 and transverse draw ratios of 3.0–3.4 is performed at 90–110°C for MD preheat and 100–120°C for TD stretching. Heat setting at 220–240°C while maintaining width improves dimensional stability; thermal shrinkage measured by ASTM D1204 at 150°C for 30 min must remain below 1.0–1.5% for flexible electronics substrates. Low-molecular cyclic oligomers migrate to the die lip during extrusion and form deposits after 30–50 h of continuous operation; melt filtration with 15–25 μm absolute screens and periodic die-lip cleaning are therefore required. Film haze measured by ASTM D1003-13 below 2–5% for optical grades requires control of catalyst residues, oligomer content, and film surface roughness. Dielectric capacitor film with thickness of 2–3 μm is tested for dielectric strength by IEC 60243-1 above 200 V/μm, while moisture barrier is measured by ASTM F1249-20 at 38°C/90% RH.

When PTA-Based DOTP Plasticizer Replaces Orthophthalates in PVC Insulation

Dioctyl terephthalate is synthesised from Weilian Chemical PTA and 2-ethylhexanol in a batch or continuous esterification reactor. The alcohol-to-PTA molar ratio is set at 2.2–2.5:1, and tetrabutyl titanate or tetra-n-butyl titanate catalyst is charged at 0.05–0.30 wt% of PTA. Reaction temperature is ramped from 180°C at the start of water evolution to 230–240°C at acid value endpoint; vacuum below 10 kPa is applied in the final stage to remove unreacted 2-ethylhexanol and drive the acid value below 0.05 mg KOH/g by ASTM D1045-14. The resulting DOTP is neutralised with aqueous sodium carbonate, washed, steam-stripped at 150–180°C under 1–2 kPa, and filtered through 5–10 μm polishing cartridges before transfer to storage.

In flexible PVC compounding for wire and cable insulation, DOTP is added at 40–80 phr with suspension-grade PVC resin, calcium-zinc stabiliser, and calcined kaolin modifier. The dry blend is processed in a twin-screw extruder with L/D 32:1–40:1 at barrel temperatures from 150–175°C; plasticiser absorption at this stage determines output and melt pressure. DOTP-plasticised compounds exhibit volume resistivity by ASTM D257-14 above 1 × 10¹¹ Ω·cm at 20°C, tensile elongation by ASTM D638-14 above 250%, and cold bend performance without cracking at -20°C depending on plasticiser content. Unlike orthophthalates, DOTP remains outside the SVHC candidate list under REACH Regulation 1907/2006, and its use is assessed under RoHS Directive 2011/65/EU for wiring applications. However, dielectric loss and water resistance may be slightly less favourable than DOP in certain high-voltage formulations, so a full compound revalidation is required when replacing orthophthalates.

PBT Engineering Resin Compounding via Direct PTA–BDO Esterification

Direct esterification of PTA with 1,4-butanediol in a titanium-catalysed melt process is the preferred route to polybutylene terephthalate when low residual methanol content is not required. The initial esterification is run at 220–235°C with a BDO-to-PTA molar ratio of 1.3–1.5:1; the formation of tetrahydrofuran from acid-catalysed dehydration of BDO is suppressed by maintaining a minimum reaction pressure of 1.5–2.0 bar(a) and refluxing BDO back to the reactor. Tetrabutyl titanate catalyst at 40–100 mg/kg Ti accelerates esterification and subsequent polycondensation at 250–260°C under vacuum <1 mbar. The final melt has an intrinsic viscosity of 0.8–1.0 dL/g measured in a phenol/tetrachloroethane 60/40 solvent system, with terminal carboxyl end groups of 15–30 meq/kg.

For injection moulding grades, PBT is compounded with glass fibre at 15–30 wt%, flame retardant packages based on brominated epoxy or alkyl phosphinate at 10–20 wt%, and nucleating agents in a co-rotating twin-screw extruder with L/D 32:1–40:1. Screw speed of 300–500 min⁻¹ and melt temperature of 245–265°C are used; excessive residence time above 270°C promotes THF regeneration and molecular weight loss. The pellets are dried at 120–140°C for 4 h to below 0.02% moisture before moulding at 240–260°C with a mould temperature of 60–80°C. Low mould temperatures below 40°C produce low crystallinity, poor dimensional stability, and reduced comparative tracking index; CTI by IEC 60112 generally falls between 200 V and 400 V depending on flame-retardant system. Glass-filled PBT tensile strength by ISO 527-2:2012 typically reaches 90–120 MPa, and flexural modulus reaches 7–9 GPa. Hydrolysis resistance at 85°C/85% RH for aging 1000 h is tested by retention of tensile strength; PBT compounds usually retain 70–80% of initial strength when stabilised with carbodiimide.

Low Carboxyl End-Value Is Maintained by Staged PTA Addition in Unsaturated Polyester Kettles

In unsaturated polyester resin kettles, PTA is used as a replacement for phthalic anhydride to increase polymer backbone rigidity and reduce raw-material volatility. The esterification is staged: maleic anhydride and propylene glycol are added first at 150–170°C, then PTA is added at 190–220°C because its higher melting point and two carboxyl groups require higher reaction temperature. The esterification catalyst, such as dibutyltin oxide or butyl stannoic acid, is charged at 0.02–0.10 wt% of total resin charge. Esterification continues until acid value by ISO 2114:2000 drops to 15–30 mg KOH/g for flexible resins or 5–15 mg KOH/g for rigid structural grades. The melt is then cooled and cut with styrene at 30–40 wt% to final viscosity of 300–600 mPa·s measured by ISO 3219:1993 at 23°C.

PTA-containing unsaturated polyesters provide a higher glass transition temperature and tensile modulus after cure than orthophthalic-based resins. Casting validation uses 1.0–1.5 phr methyl ethyl ketone peroxide and 0.2–0.5 phr cobalt naphthenate at 25°C; gel time determined by ISO 2535:2001 is typically 15–30 min. Cured neat resin tensile modulus measured by ISO 527-2:2012 can exceed 3.0–3.5 GPa for rigid grades. However, PTA-based unsaturated polyester resins are more prone to haze and styrene incompatibility when acid value remains above 20 mg KOH/g; the resin must be cooled below 80°C before styrene dilution to avoid runaway exothermic polymerisation.

Aromatic polyester polyols are prepared by depolymerising PTA-based PET production scrap or by direct condensation of PTA with diethylene glycol and polyols at 235–245°C in a nitrogen-sparged reactor. When PTA is directly charged with DEG at a 2.3–2.8:1 DEG/PTA molar ratio, water is removed through a packed column, and the reaction is continued until hydroxyl number by ASTM D4274-21 falls to 200–350 mg KOH/g and acid value to 0.5–2.0 mg KOH/g. The product is then cooled to 90–110°C and transferred through a 10–20 μm cartridge filter. Viscosity by ASTM D4878-15 is typically 2000–6000 mPa·s at 25°C, and water content by ISO 15512:2019 is maintained below 0.10%. Aromatic polyester polyols with high terephthalate content increase compressive strength and char yield in rigid polyisocyanurate foam; laminator panels are produced by reacting the polyol with polymeric MDI at an isocyanate index of 250–300, with pentane or HFO blown boardstock meeting EN 13165 thermal conductivity below 0.022–0.028 W/(m·K) at 10°C mean temperature. Operational boundaries include storage under dry nitrogen at 50–70°C to prevent crystallisation, and avoidance of zinc or amine catalysts in the polyol storage tank because these accelerate transesterification and shift hydroxyl number during ageing.